Internet on LAN

For sometime now, I've realised that the internet on my LAN network has been running slow. We have T1 line from our ISP. I did run several internet speed test and on the average my download speed is 0.67Mbs and upload speed is about 0.90Mbs.
Our T1 line from our ISP should give us about 1.5Mbs download and upload. I've made several calls to my ISP, and as usual they claim it has nothing to do from their end. I've ran these internet speed test when there was no one on the LAN and the speed results did not change. My network is domain network with about 15 PCs. No changes has been done to the network recently.
Is there anything I could do again to prove to my ISP that the issue is with them? OR is there any network tool (I will purchase if I have to) that I can run on my network that will tell me if the issue is on my network, or if there is a PC on the network that is taking much of the bandwidth , infested with viruses or doing some sort of P2P sharing/downloading on the network? I do have up to date antivirus running on all PC's on my LAN.

I do not think the internet will work at all if it is a twork looping issue. Checked all cable connections from switch to router and all is rightly connected.
Will try snifing with wireshark and advise offered by NetExpert-Warzawa and update you all soon.

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If you want to know more about traffic your computers generate, use Trafmeter - http://trafmeter.com/
Wireshark shows packets. It is hard to see a larger picture. Trafmeter shows an amount of traffic. You can filter by IPs, ports etc.

You must connect it in a way it can listen to all traffic to the Internet. It could be a hub (not switch) next to the router or a switch that allows a monitor port.

Hi NetExpert-Warszawa,
I am confused as to how you want me to distribute my points. If you had read my closing remarks, I had used a different network tool to resolve my issue. I did appreaciate your help that was why I decided to offer something for it.
If you believe, I did not treat you fairly, let me know and will reward you all points if that is going to make you happy.

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